1. Field of the Invention
The present invention relates to a power supply apparatus for driving a capacitive load, and more particularly, to a power supply apparatus capable of generating a high voltage without transformers. This power supply apparatus can be suitably used to supply high voltages to components of the image forming apparatus, such as those for a developing unit that develops a toner image on a photoconductor member and for a charger that charges the photoconductor member, and a bias voltage for a cleaner.
2. Description of the Related Art
Conventionally, the following is known as an electrophotographic image forming apparatus to which the power supply apparatus of the above-mentioned type is applied (hereinafter referred to as first conventional art for convenience' sake). The surface of a photoconductor drum is evenly charged at a given voltage by a primary charger. Then, an image is formed on the surface of the photoconductor drum by exposure, so that an electrostatic latent image corresponding to the exposed image can be formed thereon. The electrostatic latent image formed on the photoconductor drum is developed by a developing unit, this resulting in a toner image. The toner image formed on the photoconductor drum is transferred onto a transfer sheet by charging of an image transfer charger. The transfer sheet on which the toner image has been formed is separated from the photoconductor drum by charging of a separator charger. Then, the image forming process ends with the step of fixing the toner image on the transfer sheet by a fixing or fusing unit.
For example, a color image forming apparatus equipped with four developing units used to sequentially form toner images of four colors on the photoconductor drum while the drum makes four turns is required to develop the toner image of color of interest without disturbing the previously developed toner image(s) of color(s). From this viewpoint, a high-voltage power supply apparatus is used which supplies, during development, one of the four developing units with a DC development bias voltage with an AC voltage necessary for enabling excellent development being superimposed thereon, while supplying the three remaining developing units with a DC voltage that prevents toner from being deposited on the photoconductor drum.
This type of high-voltage power supply apparatus is disclosed in, for example, Japanese Laid-Open Patent Application Publication No. 6-197542, and is now illustrated in FIG. 9. Referring to FIG. 9, the high-voltage power supply apparatus is a high-voltage ac power unit for the developing unit. A dc voltage Vi applied to the primary winding N1 of a stepup transformer T is turned ON/OFF, this resulting in an induced ac voltage across the secondary winding N2. The ac voltage may be applied, as a bias voltage, to the developing unit that functions as a capacitive load. However, the use of the transformer T of the high-voltage ac power unit 10 has a limited usable frequency on the high-frequency side and is unsuitable for high-speed operation. In addition, the use of the transformer T makes it difficult to realize downsizing and weight saving.
The following is known. An alternating signal that serves as a switching pulse of 20 kHz is applied to the primary winding of the transformer T, and an induced dc voltage developing across the secondary winding is subjected to a voltage doubler rectifier. The rectified high voltage may be varied by PWM (Pulse Width Modulation). However, the above-mentioned dc power supply employs the transformer and the same problems as those mentioned before.
The above-mentioned Japanese Laid-Open Patent Application Publication No. 6-197542 also discloses another power supply, which can generate a high voltage utilizing resonance-based switching. FIG. 10 shows this type of power supply, which is called ac bias power supply apparatus. Referring to FIG. 10, an ac bias power supply apparatus 20 includes an inductor L1, a first bias circuit and a second bias circuit. The inductor L1 is connected to a capacitive load in series and forms an LC series resonance circuit together with the capacitive load. The first bias circuit includes a switching circuit SW1 and a diode D1. The second bias circuit includes a switching circuit SW2 and a diode D2. The output voltage can be controlled by adjusting the biasing times of the first and second bias circuits. The switching circuit SW1 forwardly biases the LC series resonance circuit and has a capability of controlling the biasing time. The diode D1 recovers series-resonance energy remaining after biasing by the switching circuit SW1. The switching circuit SW2 backwardly biases the LC series resonance circuit and has a capability of controlling the biasing time. The diode D2 recovers series-resonance energy remaining after biasing by the switching circuit SW2. With the above-mentioned circuit configuration, the power consumed in the capacitive load does not depend on the capacitance thereof, so that power can be supplied to the load efficiently.
There is another resonance-based power supply circuit, which is disclosed in Japanese Laid-Open Patent Application Publication No. 7-107737. This is illustrated in FIG. 11. A resonance-based power supply circuit 30 shown in FIG. 11 has a resonance circuit made up of an inductor L1 and a capacitor C4. A transistor Q3 for oscillation is connected to the resonance circuit, to which voltage doubler rectifier circuits 31 and 32 are connected. The rectifier circuit 31 includes capacitors C11 and C12 and diodes D11 and D12. Similarly, the rectifier circuit 32 includes capacitors C21 and C22 and diodes D21 and D22. By turning on/off the transistor Q3 for making oscillation, a resonance voltage VL1 is generated by the resonance circuit, and is doubled by the voltage doubler rectifier circuits 31 and 32. A triac Q11 for use in output switching is connected to the anode of the diode D11 of the voltage doubler rectifier circuit 31. Similarly, a silicon-controlled rectifier (thyristor) Q21 for use in output switching is connected to the cathode of the diode D21. By turning on/off the switching means of Q11 and Q21, the output voltage is selectively generated at output terminals OUT1 and OUT2.
The high-voltage power supply circuit 30 does not employ any transformer and instead uses the parallel resonance circuit between the power supply and the transistor Q3 serving as the switching element. Thus, the voltage waveform that swings over the positive and negative sides can be output via the voltage doubler rectifier circuits 31 and 32, so that positive and negative dc voltages can be generated.
However, the circuit configuration shown in FIG. 10 has the following disadvantages. The LC series resonance circuit of the ac bias power supply apparatus 20 that utilizes resonance is used to increase the impedance of the load and reduces the loss. Thus, unless the input voltage is increased or the stepup ratio of the transformer is increased, a sine wave having an amplitude that is large as compared to the load cannot be obtained. Further, the use of a transformer having a large stepup ratio may degrade the efficiency of the power supply apparatus.
The power supply circuit 30 shown in FIG. 11 does not have any transformer. However, the transistor Q3 is required to have a high breakdown voltage and operate at high speed in order to obtain a high-voltage, high-frequency ac output. In practice, it is therefore difficult to generate a high-frequency ac output. As is described in Japanese Laid-Open Patent Application Publication No. 7-107737, a flyback voltage may contribute to generating a high voltage having a sufficient pulse width. However, this needs a high input voltage in order to obtain a sine wave having a large amplitude.